Controlled Impedance Pcb Manufacture

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Controlled impedance PCB

Controlled impedance PCB Manufacture - As PCB signal switching speeds increase today's PCB designer needs to understand and control the impedance of PCB traces. With the short signal transition times and high clock rates of modern digital circuitry, PCB traces need to be considered not as simple connections but as transmission lines.

What is controlled impedance?

Probably the most common example of a controlled impedance component is the download (or feeder) connecting a receiving aerial to a wireless or television set. Aerial feeder leads usually take the form of "flat twin" cable (commonly supplied with VHF broadcast receivers) or low-loss coaxial cable. In both cases the impedance of the feeder is controlled by the physical dimensions and material of the cable.

So why do we need to control impedance?

The receiving aerial possesses natural, or characteristic, impedance and electrical theory shows that for the aerial to transfer maximum power to the set (and to ensure the integrity of the electrical signal) the impedance both of the feeder and the receiver should match ...
... that of the aerial. In other words the signal should ideally be presented with a constant impedance as it travels from its source to its destination. Where a mismatch occurs only part of the signal will be transmitted; the rest will be reflected toward the source (this degrades the signal). Cable designers therefore take great care to ensure the accuracy and consistency of the cable dimensions and material characteristics. At high signal switching speeds, the electrical properties of the cable, such as the capacitance and inductance, must be taken into account, and cables can no longer be considered as simple wires. Cables designed for high signal speeds where these factors are taken into consideration are referred to as transmission lines.

Controlled impedance on PCBs

Similarly, as the speed of signal switching on a PCB increases, the electrical properties of the traces carrying signals between devices become increasingly more important. The impedance of a PCB trace is controlled by its configuration dimensions (trace width and thickness and height of the board material) dielectric constant of the board material.

As with a cable, when the signal encounters a change of impedance arising from a change in material or geometry, part of the signal will be reflected and part transmitted. These reflections are likely to cause aberrations on the signal which may degrade circuit performance (e.g. low gain, noise and random errors). In practice board designers will specify impedance values and tolerances for board traces and rely on the PCB manufacturer to conform to the specification.

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